My wife and I used them on a 2 week trek earlier this month. I was originally interested in theem as they seemed to offer a solution to my artists wifes needs for carrying and quickly accessing small sketchbooks, pencils etc along with her camera and binoculars.
I found mine gradually carrying more and more as I realised what I'd like to access on the move - binoculars, sun glasses, snack bars, passport, compass, spare batteries (I keep my camera in a waist band pouch), sun cream, midge net....... Too much however and they bulge and you stand a chance of dropping something while trying to find what you need 
Luckily it didn't rain at all through the 2 weeks but we had tried them in downpours on training trips previously however. Things generally seemed to stay dry inside although I guess they can't be classified as waterproof.
You attach to straps to the rucksack onto which clips the bag. The straps can be easily removed and turned into a shoulder strap for using the bag by itself.
However, I found the down side was putting them on / taking them off. I clip mine onto my rucksack shoulder straps no problem, but find it fiddly to get the elasticated straps at the bottom correct - they are hidden from view by the bag itself when fitting. The bottom straps are necessary though, otherwise the bag swings about too much.
I thought the map pocket was a good idea at first, but I do have to partly unclip the pack to be able to view the map!
All in all, for something which I believe is designed for adventure racers, we have found them useful and will be using them again next week in Scotland.